This invention relates to a highly durable carbon powder-containing refractory for steel casting and more particularly, to a highly durable carbon-containing refractory nozzle system of a casting machine and a process for producing such a refractory nozzle system.
Of late, with the aim to increase the yield rate and reduce the cost in the production of steel castings, the so-called continuous charge casting system has been increasingly employed in place of the so-called batch charge system and particularly, in these last years the steel casting operation has been in most cases consecutively performed from several charges up to several ten charges.
In a steel casting machine, the refractory nozzle is employed between the ladle tundish and between the tundish and mould, respectively and serves to prevent oxidation of molten steel and regulate the flow direction of the molten steel. Thus, the refractory nozzle is a quite important refractory component in the steel casting machine.
However, when the refractory nozzle is employed in the continuous casting operation referred to hereinabove, since the nozzle tends to be submerged in the molten steel pool for a fairly long time period, the portion of the nozzle which is designed to come to contact with a highly erosive powder layer such as silica or alumina which is spread over the top of the molten steel pool is attacked by the erosive powder.
In order to protect the particular portion of the refractory nozzle from the erosive material, various attempts have been made up to date. The conventional attempts are (1) the formation of the nozzle with Al.sub.2 O.sub.3 and graphite, (2) the mounting of a protective ring on the powder contacting portion of the nozzle brick and (3) the simultaneous formation of the powder contacting portion and the rest of the refractory nozzle by the use of a high erosion resistance material (for example, zirconia or the like) and steel, respectively, to thereby reinforce the refractory nozzle powder contacting portion. However, none of these conventional attempts have been found perfectly satisfactory. Although the attempt (3) has been extensively studied and widely practiced because the attempt can use the expensive material such as zirconia with a high efficiency, since the nozzle is formed of a combination of different materials, this nozzle encounters difficulty in the formation of the same. In addition, the powder contacting portion of the nozzle formed of the different material combination can not be always maintained in contact with the powder layer over the molten steel pool because the refractory nozzle moves upwardly and downwardly in the casting operation to thereby render some portion of the refractory nozzle other than the powder contacting portion thereof to be exposed to the erosive powder layer which may result in giving damage to the area adjacent to the powder contacting portion of the nozzle and thus, the attempt has been found not perfectly satisfactory.
When a long nozzle the tip of which is adapted to be always submerged in the molten steel pool during the casting operation is employed, the submerged tip of the nozzle is attached by the heat of the molten steel flowing down through the refractory nozzle and wears off prematurely to thereby reduce the designed service life of the nozzle. Furthermore, when a submerged nozzle is used, alumina deposit from the molten steel tends to adhere to the orifices in the nozzle to clog up them which may also reduce the service life of the nozzle.
In order to prevent the nozzle orifices from being clogged up by the alumina deposit from the molten steel, it has been proposed to blow gas such as argon gas into the orifices. However, in the conventional refractory nozzle formed of a combination of Al.sub.2 O.sub.3 and graphite, the alumina deposit cannot be perfectly removed from the orifices and such gas blowing has been found not fully effective. And the gas blowing tends to disturb the molten steel pool to thereby accelerate the wearing-off of the refractory nozzle. The wearing-off progresses from the walls of the refractory nozzle orifices into the body of the nozzle.
Thus, there has been a demand for a nozzle for use in the continuous steel casting operation which can effectively solve the problem of orifice clogging-up by alumina deposit without the blowing of gas or by full realization of the effects of gas blowing and that of wearing-off of the nozzle.
A patent has been granted to the formation of a highly durable nozzle for the continuous steel casting operation with a combination of Al.sub.2 O.sub.3 and graphite with zircon or zirconia added thereto. However, since zircon or zirconia is employed in its original form, such additive cannot be perfectly and uniformly distributed within the Al.sub.2 O.sub.3 and graphite combination. In addition, since zircon and zirconia have a higher fire resistance than those of the other components of the refractory nozzle, such additive materials take a rather long time period until they form a highly viscous glass film through their reaction with the other refractory nozzle components and the other components tend to be adversely affected by the heat of molten steel before the glass film is formed.
And in order to prevent the refractory nozzle orifice clogging-up problem, although it has been proposed to add silica in an increased amount to the Al.sub.2 O.sub.3 and graphite combination, the addition of such increased amount of silica is still not able to appreciably reduce the possibility of refractory nozzle orifice clogging-off. And it is difficult to increase the amount of the additive in proportion to the amount of alumina deposit to prevent wearing-off and orifice clogging-up whereby the additive cannot be added in an amount to cope with variation in the type of steel material to be cast.